Bottom Line:
Treatment of EGFR-mutant lung cancer with erlotinib results in dramatic tumor regression but it is invariably followed by drug resistance.In characterizing early transcriptional changes following drug treatment of mutant EGFR-addicted cells, we identified the stem cell transcriptional regulator SOX2 as being rapidly and specifically induced, both in vitro and in vivo.Together, these observations point to a physiological feedback mechanism that attenuates oncogene addiction-mediated cell death associated with the withdrawal of growth factor signaling and may therefore contribute to the development of resistance.

ABSTRACTTreatment of EGFR-mutant lung cancer with erlotinib results in dramatic tumor regression but it is invariably followed by drug resistance. In characterizing early transcriptional changes following drug treatment of mutant EGFR-addicted cells, we identified the stem cell transcriptional regulator SOX2 as being rapidly and specifically induced, both in vitro and in vivo. Suppression of SOX2 sensitizes cells to erlotinib-mediated apoptosis, ultimately decreasing the emergence of acquired resistance, whereas its ectopic expression reduces drug-induced cell death. We show that erlotinib relieves EGFR-dependent suppression of FOXO6, leading to its induction of SOX2, which in turn represses the pro-apoptotic BH3-only genes BIM and BMF. Together, these observations point to a physiological feedback mechanism that attenuates oncogene addiction-mediated cell death associated with the withdrawal of growth factor signaling and may therefore contribute to the development of resistance.

fig3s1: Erlotinib treatment results in induction of SOX2 in vivo.Quantitative immunohistochemistry for SOX2 was performed in FFPE sectionsfrom mice xenografted subcutaneously with HCC827 cells and treated with asingle oral dose of erlotinib (red in dot plot) or carrier (green).Because HCC827 cells possess a degree of basal SOX2 in vivo, the averagemean fluorescence of tumor cells from each xenograft was used todetermine significance. p < 0.0001 for the comparison oferlotinib-treated vs control (Analysis of Variance to control for thevariation among individual mice, N = 12,893–16,140 cells).Source data are included as Figure 3—source data 2.DOI:http://dx.doi.org/10.7554/eLife.06132.024

Mentions:
To test the physiological significance of SOX2 induction following withdrawal ofmutant EGFR signaling, we first made use of mouse tumor models. The effectiveness ofEGFR inhibitors in treating patients with EGFR-mutant NSCLC is well modeled in mousexenograft assays, where oral administration of erlotinib for a few days is sufficientto cause massive regression of established tumors. We generated PC9 cell-derivedsubcutaneous tumors in nude mice and treated these with a single oral dose of 100mg/kg erlotinib when the tumors had reached approximately 500 mm3,harvesting tumors 24 hr after treatment. Immunohistochemical (IHC) analysis revealedminimal SOX2 expression in mock-treated xenografts (mean 0.2 SOX2+ nuclei perfield), but clearly increased (and heterogeneous) SOX2 positive cells after a singledose of erlotinib (mean: 7.4 SOX2+ nuclei per field, N = 147–151fields, p < 0.0001) (Figure 3). Similarstudies in HCC827 cell-derived xenografts revealed low (but detectable) levels ofSOX2 expression in mock-treated tumors; again, a single oral dose of erlotinibincreased both the number of SOX2-positive cells and the level of SOX2 expression pernucleus (Figure 3—figure supplement1). Thus, in a physiological setting that mimics the initial therapeuticresponse to EGFR inhibitors in EGFR-mutant NSCLC, treated cancercells rapidly induce SOX2.10.7554/eLife.06132.021Figure 3.SOX2 is induced by erlotinib in a subset of EGFR-mutant cells invivo.

fig3s1: Erlotinib treatment results in induction of SOX2 in vivo.Quantitative immunohistochemistry for SOX2 was performed in FFPE sectionsfrom mice xenografted subcutaneously with HCC827 cells and treated with asingle oral dose of erlotinib (red in dot plot) or carrier (green).Because HCC827 cells possess a degree of basal SOX2 in vivo, the averagemean fluorescence of tumor cells from each xenograft was used todetermine significance. p < 0.0001 for the comparison oferlotinib-treated vs control (Analysis of Variance to control for thevariation among individual mice, N = 12,893–16,140 cells).Source data are included as Figure 3—source data 2.DOI:http://dx.doi.org/10.7554/eLife.06132.024

Mentions:
To test the physiological significance of SOX2 induction following withdrawal ofmutant EGFR signaling, we first made use of mouse tumor models. The effectiveness ofEGFR inhibitors in treating patients with EGFR-mutant NSCLC is well modeled in mousexenograft assays, where oral administration of erlotinib for a few days is sufficientto cause massive regression of established tumors. We generated PC9 cell-derivedsubcutaneous tumors in nude mice and treated these with a single oral dose of 100mg/kg erlotinib when the tumors had reached approximately 500 mm3,harvesting tumors 24 hr after treatment. Immunohistochemical (IHC) analysis revealedminimal SOX2 expression in mock-treated xenografts (mean 0.2 SOX2+ nuclei perfield), but clearly increased (and heterogeneous) SOX2 positive cells after a singledose of erlotinib (mean: 7.4 SOX2+ nuclei per field, N = 147–151fields, p < 0.0001) (Figure 3). Similarstudies in HCC827 cell-derived xenografts revealed low (but detectable) levels ofSOX2 expression in mock-treated tumors; again, a single oral dose of erlotinibincreased both the number of SOX2-positive cells and the level of SOX2 expression pernucleus (Figure 3—figure supplement1). Thus, in a physiological setting that mimics the initial therapeuticresponse to EGFR inhibitors in EGFR-mutant NSCLC, treated cancercells rapidly induce SOX2.10.7554/eLife.06132.021Figure 3.SOX2 is induced by erlotinib in a subset of EGFR-mutant cells invivo.

Bottom Line:
Treatment of EGFR-mutant lung cancer with erlotinib results in dramatic tumor regression but it is invariably followed by drug resistance.In characterizing early transcriptional changes following drug treatment of mutant EGFR-addicted cells, we identified the stem cell transcriptional regulator SOX2 as being rapidly and specifically induced, both in vitro and in vivo.Together, these observations point to a physiological feedback mechanism that attenuates oncogene addiction-mediated cell death associated with the withdrawal of growth factor signaling and may therefore contribute to the development of resistance.

ABSTRACTTreatment of EGFR-mutant lung cancer with erlotinib results in dramatic tumor regression but it is invariably followed by drug resistance. In characterizing early transcriptional changes following drug treatment of mutant EGFR-addicted cells, we identified the stem cell transcriptional regulator SOX2 as being rapidly and specifically induced, both in vitro and in vivo. Suppression of SOX2 sensitizes cells to erlotinib-mediated apoptosis, ultimately decreasing the emergence of acquired resistance, whereas its ectopic expression reduces drug-induced cell death. We show that erlotinib relieves EGFR-dependent suppression of FOXO6, leading to its induction of SOX2, which in turn represses the pro-apoptotic BH3-only genes BIM and BMF. Together, these observations point to a physiological feedback mechanism that attenuates oncogene addiction-mediated cell death associated with the withdrawal of growth factor signaling and may therefore contribute to the development of resistance.